Because of the special structures of the 137Cs source,the Y ray emitted by the radioactive source contains backscattering photons. And using this type of storage source in dosimeter's calibration, there will be some deviations. In this paper, a three dimensional model of the multi-source illuminator is established, which is used to calculate and analyze the effects of the walls to the backscattering counting. Then the 137Cs energy spectrum is measured, and two methods to deduct Compton plateau are used to calculate the ratio of the backscatter...